Bolton is a hawk’s hawk who has repeatedly said he believes the United States has the right to take the first shot to handle potential threats.

Several commentators have expressed concern that Bolton’s bellicose approach will only encourage Trump’s more aggressive foreign policy instincts and could help lead the United States into what might be otherwise avoidable military conflicts.

Here are some facts from Bolton’s past that help show why his appointment has made some people nervous.

He thinks pre-emptive war with Iran and North Korea is the right call

Bolton is strongly opposed to the Iran nuclear deal and said on Fox News that the United States “has no other option” than to bomb the country. According to The American Conservative, “he has been obsessed for many years with going to war against the Islamic Republic.”

He still thinks invading Iraq was the right call

In the run-up to the U.S. invasion of Iraq, Bolton joined other administration officials in expressing confidence that Saddam Hussein had weapons of mass destruction and that Iraqis would welcome the U.S. overthrow of the regime.

Both those predictions turned out to be incorrect, but unlike his new boss, Bolton still believes the invasion was the right decision.

“I still think the decision to overthrow Saddam was correct,” Bolton told the Washington Examiner in 2015. “I think decisions made after that decision were wrong, although I think the worst decision made after that was the 2011 decision to withdraw U.S. and coalition forces.”